[0001] The present invention relates to crash-survivable protective enclosures for flight
recorders used in aircraft and other vehicles operating over land and water, which
are capable of withstanding the high impact, shock and mechanical penetration forces
that commonly occur in a crash and the high temperatures, flames, and heat that often
develop after a crash. The enclosure is designed to resist damage to solid state memory
devices contained therein, even if the enclosure is subjected to immersion in fresh
water or salt water for a prolonged period.
[0002] US-A-4694119 discloses crash-survivable flight data recorders in which a solid state
electronic memory device is encapsulated in a synthetic organic wax or pentaerythritol
which exhibits a phase transition at a temperature above the normal operating temperature
of the memory device, and below the maximum acceptable peak temperature for that device.
SUMMARY OF THE INVENTION
[0003] According to the present invention, a crash-survivable protective enclosure for thermally
protecting one or more heat sensitive devices from a high temperature environment
comprises a) an outer housing having interior surfaces defining an interior cavity
for containing one or more heat sensitive devices; and
b) a heat absorption composition comprising a polyoxymethylene polymer. The composition
occupies at least a portion of the interior cavity and substantially surrounds the
heat sensitive devices. The polymer exhibits endothermic decomposition when the enclosure
is subjected to the high temperature environment, and the composition absorbs heat
from the high temperature environment during the endothermic decomposition of the
polymer.
[0004] This aspect, and other aspects, of the present invention are described further in
the detailed description of the preferred embodiments of the invention which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005]
Figure 1 is a graphical representation of the time versus temperature curve for the polyoxymethylene
polymer compositions used in the present invention as well as conventional heat absorption
compositions employed in thermally protected enclosures such as flight data recorders.
Figure 2 is a cross-section of a thermally protected enclosure according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0006] As used herein, all terms have their ordinary meaning in the art unless otherwise
indicated. All percentages are percent by weight unless otherwise indicated.
[0007] The enclosure of the present invention involve the use of certain polymer compositions
which are characterized in that they absorb heat from a high temperature environment
and thereby dissipate heat away from heat sensitive devices which are exposed to the
high temperature environment. Such compositions are particularly useful in thermally
protected enclosures such as flight data recorders. The compositions used in the the
present invention are designed to possess greater heat absorption, and thus heat dissipating,
characteristics than conventional heat absorption compositions which are currently
employed in flight data recorders to absorb and dissipate heat away from heat sensitive
devices. The heat absorption characteristics of the composition are useful for providing
thermal insulation to heat sensitive devices within the enclosure in a fire or other
high temperature environment. In accordance with the present invention, the composition
absorbs heat from the high temperature environment during the endothermic decomposition
of the polyoxymethylene polymer. The absorption of heat from the high temperature
environment effectively dissipates the heat away from the heat sensitive devices by
controlling the temperature rise within the interior compartment of the flight data
recorder. As a result, the temperature to which the heat sensitive devices are exposed
does not rise to a level at which the heat sensitive devices will be damaged even
though the temperature to which the flight data recorder is exposed is significantly
higher than the temperature at which damage to the heat sensitive device will occur.
[0008] The temperature at which the endothermic decomposition occurs is selected to be:
(a) above the peak temperature encountered under normal operating conditions; and
(b) at or below the peak temperature objective for the heat sensitive devices within
the enclosure which are desirously protected from the high temperature environment.
[0009] The compositions employed in the methods and enclosures of the present invention
include a polyoxymethylene polymer having the general formula:
H-(-O-CH
2-)
n-OH
where n is a number from 8 to 100 inclusive.
[0010] Polymers having the forgoing general formula are commercially available in the art
and may be known as "paraformaldehyde". Another polymer having the foregoing structural
formula is commercially available under the tradename DELRIN® from DuPont. Those skilled
in the polymer art may readily identify other suitable commercially available polymers
having the foregoing general formula by reference to any of several chemical reagent
supplier catalogues.
[0011] Certain commercially available polyoxymethylene polymers may be stabilized to prevent
unzipping. Accordingly, when such polymers are employed in the methods and compositions
of the present invention, it may be desirable to include, in the composition, an acid
catalyst capable of initiating the degradation or unzipping of the stabilized polyoxymethylene
polymer. The required decomposition rate, which is a function of the heat flow-rate
into the interior of the enclosure, will determine whether a catalyst is needed. In
addition, the heat-flow rate will also determine how the catalyst should be included
in the composition. If only a little catalytic activity is required, the catalyst
may be applied as a dry power to the surface of the heat absorbing composition. If
greater catalytic activity is required, it may be beneficial to disperse the acid
catalyst throughout the polymer composition. Suitable acid catalysts which may be
employed to catalyze the decomposition of the polyoxymethylene polymer in the high
temperature environment include, catalysts which generate acids upon exposure to heat.
Specific examples of acid catalysts include benzoic acid, diphenyliodonium triflate
and triphenylsulfonium triflate. Typically, the acid catalyst is provided in an amount
of from about 0 to about 5 percent by weight of the composition.
[0012] If desired, the heat absorption compositions may include a binder, although a binder
is not necessary. The polymers employed in the present invention are typically in
the form of a powder, flake or other particulate form, which can be molded, extruded,
or machined into a complex shape. However, a binder may be admixed with the particulate
polymer to assist in holding the polymeric particles together during the molding process.
For example it is often desirable for compositions for use in thermally protected
enclosures to be molded into the general configuration of the interior cavity of the
enclosure or smaller geometrically shaped units such as blocks and cubes. The binder
holds the aggregated powder in the desired configuration. As a further advantage,
the binder also provides shock absorption properties to the composition.
[0013] The binder is typically a wax binder. Examples of suitable wax binders include but
are not limited to polyethylene wax, polypropylene wax, oxidized polyethylene, copolymers
of ethylene and comonomer, and copolymers of propylene and comonomer. Suitable comonomers
include those monomers which are copolymerizable with ethylene or propylene, such
as linear, branched, or cyclic alkene monomers including butene, hexene, pentene,
octene, and higher homologs; acrylic acid; and vinyl acetate. These binders, including
the copolymers, are commercially available. Preferred binders include oxidized polyethylene
wax, paraffin wax, and poly(ethylene-vinyl acetate). Oxidized polyethylene wax is
currently the most preferred binder for use in the compositions of the present invention.
[0014] Typically, the composition includes
a) between about 90 percent and about 100 percent by weight inclusive of the polyoxymethylene
polymer; and
b) between about 10 percent and about 0 percent by weight of the binder. More preferably
the composition comprises about 95 percent by weight of a polymer and about 5 percent
by weight of a binder. One preferred composition includes about 99 percent by weight
of the polymer and about 1 percent by weight of the binder. In another preferred embodiment,
the composition does not include binder, and includes 100 percent by weight of the
polyoxymethylene polymer.
[0015] In those embodiments wherein a binder is included in the composition, the composition
may be prepared by manually combining the polymer and binder in a manner that provides
a relatively uniform composition. The composition may be prepared by manually combining
the two components in a manner that provides a relatively uniform composition. The
composition may be prepared by mechanically mixing, stirring or otherwise combining
the components so as to provide a blend of uniform consistency. For example, the composition
may be prepared by blending the binder into the particulate polymer component. Once
a composition of uniform consistency is achieved, the composition may be molded into
any suitable shape by using relatively low pressure. The blocks which are often desirable
for use in flight data recorders may be prepared using compression molding techniques.
For example blocks may be prepared by grinding the wax binder below room temperature
into a fine powder, mixing the powdered wax binder with the polymer at a temperature
of from between about 50 and about 95 at a pressure between about 5625 psi and about
22,500 psi. Typically, the higher temperatures and pressures are preferred in the
compression molding of the compositions of the present invention.
[0016] In a fire or other high temperature environment, the compositions of the present
invention act as a heat shield initially, by exhibiting thermal inertia. When the
composition reaches the reactivity temperature, i.e., the temperature at which decomposition
of the polymer begins, the composition acts as a heat sink by utilizing the heat energy
of the high temperature environment to initiate and maintain the endothermic decomposition
reaction of the polymer. The reactivity temperature of the composition is about 140°C.
[0017] Once the compositions reach the reactivity temperature, the endothermic decomposition
reaction begins. At temperatures equal to or greater than the reactivity temperature,
the polyoxymethylene polymer undergoes a polymer "unzipping" reaction as follows.
H-(-O-CH
2-)
n-OH _ n H
2C=O + H
2O.
In other words, the polymer decomposes back to its original monomers, namely formaldehyde.
The polymer unzipping proceeds by an endothermic reaction mechanism. The term "endothermic"
is well defined in the art and refers to reactions which require the input of heat
energy to maintain the reacting mass at a constant temperature. Thus, the heat absorption
composition absorbs heat from the high temperature environment to initiate and drive
the endothermic decomposition of the polyoxymethylene polymer. The use of the heat
energy to initiate and drive this endothermic reaction has the effect of maintaining
the maximum temperature environment within the enclosure at an acceptable level which
does not cause significant heat damage to the heat sensitive devices within the enclosure.
[0018] The formaldehyde product which results from the endothermic decomposition reaction
is flammable. The amount of formaldehyde gas released from the composition is often
significant. Without venting, the pressure could rise excessively and the heat dissipation
properties would also be lower. To avoid the potential pressure damage to the heat
sensitive devices within the enclosure as the pressure from the formaldehyde gas increases,
it is desirable to provide a vent in the enclosure to allow the escape of the generated
formaldehyde gas.
[0019] The expected gas evolution rate can be predicted from the heat flow anticipated in
the final design. The heat flow can be estimated from the wall type and thickness,
the insulation type and thickness (or thermal conduction coefficient) and the approximate
physical size of the unit. Given the heat flow, the temperature profile and gas evolution
rates can be estimated from each material's known thermochemical behavior.
[0020] Figure 1 is a graphical representation of the time versus temperature characteristics of the
compositions of the present invention as compared to sodium bicarbonate, another new
heat absorption composition, which is described in one of the Applicant's copending
Applications. As can be seen from the graph, the polyoxymethylene polymer compositions
used in the present invention absorb more heat than the sodium bicarbonate compositions,
which in turn are more efficient than conventional heat absorption compositions. The
time required for the polyoxymethylene polymer composition to reach a given temperature
in a high temperature environment is as long or longer than the time required for
the bicarbonate composition to reach the same temperature. This result demonstrates
that the polyoxymethylene polymer compositions are as effective or more effective
than the sodium bicarbonate at absorbing heat from the high temperature environment.
The compositions used in the present invention have an added advantage over conventional
silica compositions in that they will not loose water in an unconfined space. An additional
advantage over hydrated silica composition is ease of formability and subsequent handling
during the manufacturing process.
[0021] The improved heat absorption properties observed with the polyoxymethylene polymer
compositions used in the present invention as compared to conventional heat absorption
compositions is a very important advantage in applications such as flight data recorders.
In the event of a crash, the flight data recorder may be subjected to temperatures
in the range of 1100°C for up to an hour. Thus it is clearly advantageous to use a
heat absorption composition which absorbs more heat energy and thereby provides greater
protection to the heat sensitive devices contained therein.
[0022] In addition to the foregoing, the compositions used in the present invention provide
a number of advantages. The components of the composition are relatively easy to handle,
relatively low in cost, formable, and more effective at heat absorption as compared
to conventional heat absorption materials. Also, because the compositions used in
the present invention absorb heat more efficiently than conventional heat absorption
agents, flight data recorders including the compositions of the present invention
may be smaller than conventional flight data recorders without sacrificing heat absorption
protection for the heat sensitive devices contained therein.
[0023] The compositions used in the present invention are particularly useful in enclosures
for thermally protecting one or more heat sensitive devices from a high temperature
environment, such as aircraft flight data recorders which include heat sensitive devices
for recording and storing data. Flight data recorders are known in the art and are
configured to provide a record of various important aircraft performance parameters
over a predetermined time interval. Flight data recorders record performance parameters
continuously from the time the electrical system of the aircraft is activated. Because
they have a limited memory they can only hold information relating to such parameters
for the predetermined time interval. Hence, as new data is continuously collected,
it is continuously written over old data. In the event that the flight data recorder
is deactivated, which occurs if the aircraft crashes, the flight data recorder will
retain in memory the performance parameters recorded over the predetermined time interval
immediately prior to deactivation. In operation, the information stored within a memory
unit is continually supplied by additional components of the flight data recorder
system such as a data acquisition unit that receives input signals from various aircraft
sensors and systems, and processes those signals to produce signals compatible with
the recording or storage medium employed by the flight data recorder memory unit.
[0024] The compositions used in the present invention may be employed in any conventional
thermally protected flight data recorder, including but not limited to the flight
data recorders described in U.S. Patent Nos. 4,694,119 and 4,944,401 both to Gruenwegan,
U.S. Patent No. 4,600,449 to White et al., and U.S. Patent No. 5,438,162 to Thompson
et al. Generally, the thermally protected flight data recorders are one type of thermally
protected enclosures which include an outer housing, the heat sensitive devices for
recording data, and the heat absorption (i.e., thermal protecting) composition of
the present invention.
[0025] Referring to
Figure 2, the outer housing
15 of the enclosure
40 is typically constructed of a metal that exhibits a high thermal conductivity and
high resistance to crushing and piercing. An intumescent coating or paint is generally
applied to the exterior surfaces of the outer housing
15 for additional thermal insulation. The enclosure
10 may be provided in any suitable shape, and is typically substantially rectangular
in cross-section when viewed perpendicular to each of its major axes. An insulating
layer of solid material that exhibits a relatively low thermal conductivity adjoins
each interior surface of the outer housing
15 to form a rectangular interior cavity that is centrally located within the outer
housing
15. As noted above, the housing
15 may be designed to permit the escape of gaseous carbon dioxide which is produced
in the endothermic decomposition of the bicarbonate compound.
[0026] The one or more heat sensitive devices
25 employed in the enclosure
10 are located within and contained by the interior cavity of the housing
15. In a preferred embodiment of the invention, the heat sensitive devices
25 are solid state electronic memory devices for storing data which is to be recovered
from each of the solid state electronic memory devices following exposure of the enclosure
10 to a high temperature environment. Examples of such devices include semiconductor
electronically erasable programmable read-only memory circuits. A data acquisition
unit periodically supplies digital signals which are sequentially written into the
semiconductor memory circuits so that the memory circuits store a sequence of digital
words that is a time sampled data representation of the history for each parameter
being monitored. Typically, with respect to currently employed techniques, data compression
is generally employed to allow storing digital signals representative of a 15-30 minute
time history for each monitored parameter.
[0027] To provide the heat shielding necessary to protect these heat sensitive devices
25 from a high temperature environment to which the enclosure
10 is subjected, the composition
20 of the present invention is provided in the open spaces of the interior cavity of
the housing
15, substantially surrounding the heat sensitive devices
25. The composition
20 occupies at least a portion of the interior cavity. The compositions
20 used in the present invention are formable under low pressure and at temperatures
below the melting point of either component, as discussed above. Accordingly, the
composition
20 may be molded to both conform to the shape of the interior cavity and to surround
the heat sensitive devices therein.
[0028] The compositions used in the present invention do not combust within the enclosure
because the small amount of oxygen within the enclosure will be rapidly depleted or
expelled. The compositions used in the present invention also absorb radiant heat
from any residual insulation and from the interior surfaces of the enclosure. This
heat is subsequently carried outside the container and disposed of in the flame front
rather than building up inside the small cavity containing the heat sensitive devices.
[0029] In addition to their utility as heat absorbing materials, the compositions used in
the present invention, particularly in those embodiments including a binder, are also
useful in enclosures such as flight data recorders as shock absorbing materials. For
example, the enclosure may be subjected to intense shock in the area of 3400 Gs on
impact during crash conditions. Under such physical impact force, the compositions
of the present invention absorb a portion of the shock by deforming enough to divert
the impact shock wave away from the memory device, but not enough to penetrate any
surrounding insulation creating voids that might become heat flux paths to the heat
sensitive devices.
[0030] Although the essential elements of the enclosure include only the housing, the heat
sensitive devices, and the composition used in the present invention, one skilled
in the art can envision a number of modifications to the enclosure which are based
upon conventional technology in the art of flight data recorders, and are therefore
contemplated by the instant invention. For example, it may be desirable to include
as another element of the enclosure, a thermal insulating layer between the compositions
of the present invention and the interior surfaces of the interior cavity. Suitable
thermal insulating liners will be readily determinable by those skilled in the art
and include a shell-like thermal liner positioned within the interior cavity adjacent
to the interior surfaces of the interior cavity, which conforms to the geometric shape
of the interior cavity. Preferably, the thermal liner is a unitary structure formed
of a solid material that is a good thermal insulator (i.e., has a low thermal conductivity,
K) and relatively low density. Examples of such materials include but are not limited
to thermal insulators sold under the tradenames MIN-K 2000®, from Johns-Manville Co.,
of Denver CO, and MICROTHERM®, from Upton-Wirral Merseyside, England.
[0031] The foregoing is illustrative of the present invention and is not to be construed
as limiting thereof. The invention is defined by the following claims, with equivalents
of the claims to be included therein.
1. A crash-survivable protective enclosure for thermally protecting one or more heat
sensitive devices from a high temperature environment, said enclosure comprising:
an outer housing having interior surfaces defining an interior cavity, said interior
cavity for containing said one or more heat sensitive devices; and
a heat absorption composition comprising a polyoxymethylene polymer;
wherein said composition occupies at least a portion of said interior cavity and
substantially surrounds said one or more heat sensitive devices;
wherein said polymer exhibits endothermic decomposition when said enclosure is
subjected to said high temperature environment, and wherein said composition absorbs
heat from said high temperature environment during said endothermic decomposition
of said polymer.
2. The enclosure according to claim 1, wherein said one or more heat sensitive devices
is a solid state electronic memory device for storing data which is to be recovered
from each of said solid state electronic memory devices following exposure of said
enclosure to said high temperature environment.
3. The enclosure according to claim 1 or claim 2, wherein said composition is in the
form of blocks which occupy at least a portion of said interior cavity.
4. The enclosure according to any preceding claim, further comprising a thermal insulting
liner between said composition and said interior surfaces of said interior cavity.
5. The enclosure according to any preceding claim, which comprises a vent therein.
6. The enclosure according to any preceding claim and, enclosed therein, a heat sensitive
device as defined in claim 2.
7. Use of an enclosure as defined in any of claims 1 to 5, as a crash-survivable protective
enclosure for a flight recorder.
1. Aufprallsicherer Schutzbehälter für den thermischen Schutz eines oder mehrerer hitzeempfindlicher
Geräte vor einer Hochtemperaturumgebung, wobei der Behälter folgendes umfaßt:
ein Außengehäuse mit einen Innenhohlraum definierenden Innenflächen, wobei der Innenhohlraum
zur Aufnahme des einen oder der mehreren hitzeempfindlichen Geräte bestimmt ist, und
eine Hitzeabsorptionszusammensetzung aus einem Polyoxymethylenpolymer,
wobei die Zusammensetzung mindestens einen Teil des Innenhohlraums ausfüllt und das
eine oder die mehreren hitzeempfindlichen Geräte im wesentlichen umgibt,
wobei das Polymer eine endotherme Zersetzung aufweist, wenn der Behälter der Hochtemperaturumgebung
ausgesetzt wird und die Zusammensetzung während der endothermen Zersetzung des Polymers
Hitze von der Hochtemperaturumgebung absorbiert.
2. Behälter nach Anspruch 1, wobei es sich bei dem einen oder den mehreren hitzeempfindlichen
Geräten um ein elektronisches Halbleiterspeicherbauelement zum Speichern von Daten
handelt, die aus jedem der elektronischen Halbleiterspeicherbauelemente zurückgewonnen
werden sollen, nachdem der Behälter der Hochtemperaturumgebung ausgesetzt war.
3. Behälter nach Anspruch 1 oder 2, wobei die Zusammensetzung in Form von Blöcken vorliegt,
die mindestens einen Teil des Innenhohlraums ausfüllen.
4. Behälter nach einem der vorhergehenden Ansprüche, weiterhin mit einer wärmeisolierenden
Auskleidung zwischen der Zusammensetzung und den Innenflächen des Innenhohlraums.
5. Behälter nach einem der vorhergehenden Ansprüche, der darin eine Entlüftung enthält.
6. Behälter nach einem der vorhergehenden Ansprüche und darin enthalten ein hitzeempfindliches
Gerät wie in Anspruch 2 definiert.
7. Verwendung eines Behälters, wie er in einem der Ansprüche 1 bis 5 definiert ist, als
ein aufprallsicherer Schutzbehälter für einen Flugschreiber.
1. Enceinte de protection résistant aux collisions pour protéger thermiquement un ou
plusieurs appareils sensibles à la chaleur d'un environnement à température élevée,
ladite enceinte comprenant :
- un boîtier externe ayant des surfaces internes définissant une cavité interne, ladite
cavité interne pour contenir ledit un ou plusieurs appareils sensibles à la chaleur
; et
- une composition d'absorption de la chaleur comprenant un polymère polyoxyméthylène
;
dans laquelle ladite composition occupe au moins une partie de ladite cavité interne
et entoure essentiellement ledit un ou plusieurs appareils sensibles à la chaleur
;
dans laquelle ledit polymère montre une décomposition endothermique quand ladite enceinte
est soumise audit environnement à température élevée, et dans laquelle ladite composition
absorbe la chaleur dudit environnement à température élevée pendant ladite décomposition
endothermique dudit polymère.
2. Enceinte selon la revendication 1, dans laquelle ledit un ou plusieurs appareils sensibles
à la chaleur est un appareil à mémoire électronique à l'état solide pour stocker des
données qui doivent être récupérées à partir de chacun desdits appareils à mémoire
électronique à l'état solide après exposition de ladite enceinte audit environnement
à température élevée.
3. Enceinte selon la revendication 1 ou la revendication 2, dans laquelle ladite composition
est sous forme de blocs qui occupent au moins une partie de ladite cavité interne.
4. Enceinte selon l'une quelconque des revendications précédentes, comprenant en outre
un revêtement isolant thermiquement entre ladite composition et lesdites surfaces
internes de ladite cavité interne.
5. Enceinte selon l'une quelconque des revendications précédentes, qui comprend un évent.
6. Enceinte selon l'une quelconque des revendications précédentes, renfermant un appareil
sensible à la chaleur comme défini à la revendication 2.
7. Utilisation d'une enceinte comme définie dans l'une quelconque des revendications
1 à 5, comme enceinte de protection résistant aux collisions pour un enregistreur
de bord.